In general, retreading involves replacing the tread on worn tires. Retreading preserves approximately 90 percent of the material in worn tires. The material cost associated with retreading is substantially less than the cost of manufacturing a new tire, and retreading permits significant recycling.
In a retreading process, a tire casing may be first inspected, which may involve visual and/or machine-based (e.g., shearographic) inspection to identify non-visible damage and embedded debris (e.g., nails). If the casing passes the initial inspection, the worn tread may be removed through a process referred to as buffing.
Buffing typically may involve removing excess rubber to provide a substantially evenly-textured crown for receiving a pre-cured tread strip and to provide a predetermined tire casing profile. Tire casings typically may include a belt package (a package of steel belts or cables) underlying the road-engaging surface (e.g., the original tread) of the tire. Prior to retreading, the casing may be buffed, generally to a predetermined characteristic crown radius corresponding to the upper contour of the belt package. The casing may be buffed to leave only a predetermined thickness, e.g., 3/32 of an inch, of material remaining over the top belt. The shoulder of the casing may also be buffed (trimmed) to eliminate or reduce voids or patterns in the shoulder created by the original tread, and to provide, typically, a relatively continuous profile between the casing sidewalls and the crown. A worn casing from each of various models and sizes of new tires has a characteristic tire casing profile of a particular crown width, crown buffing radius and shoulder trim angle which may be created as an initial step in the buffing process.
After being buffed, the tire casing may then be examined for injuries, which may be skived and filled with a repair gum. After completion of the skiving process, the buffed surface may be sprayed with tire cement that provides a tacky surface for application of a suitable layer of bonding material, such as cushion gum. The cushion gum may be a layer of uncured rubber material, which optionally may include a low temperature vulcanizing agent and accelerator. The cushion gum can be placed over the crown. In some retreading operations, the spray cement can be omitted.
A cured tread strip, typically of a width corresponding to the width of the crown of the casing, may then be cut to the length corresponding to the casing circumference and disposed over the casing crown. Alternatively, continuous replacement treads in the shape of a ring (i.e., ring treads) may be used to retread the buffed casing. A roller pressing process, commonly referred to as stitching, may be performed on the assembly to force air from between the tread strip and casing.
After stitching the tire assembly, which includes the tire casing, the cushion gum and the tread, the assembly may be placed within a flexible rubber envelope. An airtight seal can be created between the envelope and the bead of the tire casing. The entire envelope, with the tire assembly disposed therein, can be placed within a curing chamber and subjected to elevated pressure and temperature for a predetermined period of time. The combination of exposure to elevated pressure and temperature for a duration of time may bind the cushion gum to both the tire casing and the new tire tread.
Buffing in the noted process may be performed by a buffing apparatus including a rasp and a tire mounted so that the rasp may come into contact with the tire's outer surface. An operator may perform an initial inspection process to analyze the current condition of a tire casing. The tire casing may then be mounted to the tire hub assembly. For example, the tire hub assembly may include a tire chuck with an expandable rim for accepting tire casings of various sizes. After buffing has been completed the operator may remove the tire and send it to another location for further processing.
Buffing and skiving processes take approximately 3 minutes per tire. Of the 3 minutes, the buffing process takes approximately 2 minutes, loading and unloading take approximately 15 seconds each, and skiving takes approximately 30 seconds. In addition, the rasp head must come to a complete stop after completing a buffing process, and must be restarted to buff the next tire. When operating at production rate, this corresponds to approximately 20 start/stop sequences per hour. Each start/stop sequence requires a substantial in-rush of current into the rasp motor in order to accelerate the rasp head to bring the rasp head up to operating speed. Energy is also needed to bring the rasp head to a stop. The start/stop sequences also stress and ultimately reduce the useful life of the components of the rasp head, thereby requiring regular maintenance.